Many industries have become highly dependent on products that cannot be made without using rare earth metals (REEs) (Yu Y et al., A review. J. Rare Earths, 2015, 33: 453; Chen L et al., J. Rare Earths, 2015, 33: 107). Although the importance of REEs is increasing, the price of REEs is not stable due to a large change in supply (Baldi L. et al., Energy Policy, 2014, 66:53). For example, if China, which monopolizes the supply of rare earth metals, supplies a limited amount of rare earth metals, such supply cannot meet the demand of other countries (Wubbeke J. et al., Res. Policy, 2013, 38: 384). For this reason, many trading corporations and manufacturers in the world have recognized REEs as industrially and economically important resources, and thus have been planning their strategies to secure a stable supply.
As such, it is believed that waste containing REEs as part of ensuring a stable supply can play an important role in supplying resources, and many researchers have actively conducted studies on the recovery of REEs from various types of REEs-containing wastes. Indeed, a large amount of wastes containing REEs have been produced in various industry links, including rare metal radioactive residues, hydrometallurgy residues, polishing powders, rare metal catalysts, magnetic materials, disused hydrogen storage batteries, etc. (Tao X. et al., J. Rare Earths, 2009, 27: 1096).
In the polishing powder industry, REEs are regarded as ideal polishing material, and showed increased efficiency when actually used in industrial polishing processes (Beaudry B. J. et al., Handbook on the Physics and Chemistry of Rare Earths, 1978, 1: 173). For example, rare earth-based oxides have been widely used as polishing powder for glass, semiconductors and ceramics. This is because these oxides have crystalline structures with high polishing ability, mechanical strength and abrasion resistance. In particular, the fact that cerium oxide (CeO2) is a representative rare earth-based oxide showing such characteristics has been proved by numerous studies (Ong N. S. et al., Mater. Proc. Technol., 1998, 83: 261), and in recent years, the dependency of cerium oxide in rare earth-based oxides has started to increase. For this reason, rare earth polishing powder wastes (REPPWs) that currently occurs after polishing processes contain cerium oxide (CeO2) as a main component and also contain small amounts of rare earth oxides such as La2O3, Pr2O3 and Nd2O3. However, polishing powder wastes contain not only rare earth oxides, but also polishing target residues consisting mostly of Al, Ca and Si components, which interfere with recycling of the polishing powder wastes. As mentioned above, research on the recovery of rare earths from rare earth polishing powder wastes (REPPWs), which are not easy to recycle, is urgently needed to ensure a smooth supply of REEs.
Among hydrometallurgical processes, an acid leaching process employing a sulfuric acid solution is the most general method that can recover REEs from REPPWs (Um N. et al., Mater. Trans., 2013, 54: 713). However, the acid leaching process has some drawbacks in terms of separation and recovery. Because acid leaching can dissolve not only the materials to be recovered, but also other materials, another method is needed to separate these ions after acid leaching. Namely, if REEs are dissolved in an acid solution, it is inevitably difficult to purify these REEs from non-rare earth ions (Al, Ca, etc.). Since non-rare earth ions that are present together with rare earth oxides may have similar chemical and physical properties, it is considerably difficult to separate individual elements.
Accordingly, the present inventors have made extensive efforts to solve the above-described problems occurring in the prior art, and as a result, have found that rare earth elements contained in polishing powder waste can be separated from impurities such as aluminum, silicon, calcium, magnesium and the like by an effective wet process and thus recycled, thereby completing the present invention.